1. Field of the Invention
The present invention relates to generally to magnetic recording media, and more particularly to recording media having a landing zone with an independently optimized surface texture.
Magnetic recording disks generally comprise a disk substrate having a magnetic layer and a number of underlayers and overlayers deposited thereon. The nature and composition of each layer is selected to provide desired magnetic recording characteristics. An exemplary present day disk is illustrated in FIG. 1 and comprises a non-magnetic disk substrate 10 typically composed of an aluminum alloy. Alternative substrates comprise non-metallic materials such as glass, ceramics, glass ceramic composites, carbon, carbon ceramic composites, and the like. Generally, an amorphous nickel phosphorus (Nixe2x80x94P) layer 12 is formed over each surface of disk substrate 10, typically by plating. The Nixe2x80x94P layer is hard, and imparts rigidity to aluminum alloy substrates. A chromium ground layer 14 is formed over the Nixe2x80x94P layer 12, typically by sputtering, and the magnetic layer 16 is formed over the ground layer 14. The magnetic layer 16 comprises a thin film of a ferromagnetic material, typically including an alloy of cobalt. Usually, a protective layer 18, such as a carbon film, is formed over the magnetic layer 16, and a lubricating layer 20 is formed over the protective layer.
The presence of the Nixe2x80x94P layer 12 and the chromium ground layer 14 has been found to improve the recording characteristics of the magnetic layer 16. In particular, the chromium ground layer formed over an Nixe2x80x94P layer has been found to provide enhanced coercivity and reduced noise characteristics. Such improvements are further enhanced when the Nixe2x80x94P layer is treated by mechanical texturing to create a roughened surface prior to formation of the chromium ground layer. The texturing may be circumferential or crosswise, with the preferred geometry depending on the particular composition of the cobalt-containing magnetic layer.
Such magnetic recording disk structures have been very successful and allow for high recording densities. As with all successes, however, it is presently desired to provide magnetic recording disks having even higher recording densities. Recording densities can be improved by reducing the spacing between the recording transducer (read/write head) and the magnetic disk surface while the disk is rotating. In modern magnetic recording systems, the read/write head often glides over the recording surface on an xe2x80x9cair bearing,xe2x80x9d a layer of air which moves with the rotating disk. Thus, the spacing between the read/write head and recording surface, referred to as the xe2x80x9cglide height,xe2x80x9d depends in part on the surface topology of the disk.
Surface topology affects both the magnetic recording characteristics and durability of magnetic recording media. Surface topology is often measured by surface roughness (Ra), the arithmetic average of the absolute height and depth of peaks and valleys in a profiler scan. Recording densities generally benefit from low glide heights which are associated with smooth recording surfaces having a low surface roughness. As might be expected, magnetic recording media noise, as measured in terms of bit shift, increases as roughness increases. Furthermore, certification errors per data track, the number of individual bits which exhibit less than a threshold percentage of the nominal signal strength, also increase with increasing roughness. Thus, magnetic recording characteristics generally benefit from recording surfaces having a relatively low average surface roughness.
Unfortunately, the reliability of magnetic recording systems generally improves with increased recording surface roughness. Smooth surfaces do not build up the moving layer of air over the disk""s surface required to xe2x80x9cflyxe2x80x9d the read/write head as quickly as rough surfaces. Frictional contact between the rotating disk and read/write head, called xe2x80x9cstiction,xe2x80x9d is particularly problematic during start up and stopping of the magnetic recording system, and has a profound impact on the durability of magnetic recording media.
For these reasons, it would be desirable to provide improved magnetic recording media having optimized surface topologies and methods for their fabrication. It would be particularly desirable if such recording media provided improved magnetic recording characteristics of low surface roughness without compromising the mechanical durability of magnetic recording systems. The methods should provide for texturing the substrate or layer structure of magnetic recording media without greatly increasing production costs and capital equipment requirements.
2. Description of the Background Art
U.S. Pat. No. 4,786,564 describes the texturing of a nickel phosphorus layer over an aluminum substrate to enable a read/write head to fly over the surface of the disk. U.S. Pat. No. 5,314,745 describes a magnetic recording media having a glass substrate with an optionally textured Nixe2x80x94P layer.
Magnetic recording media according to the principles of the present invention comprise a read/write head interaction surface including a contact start stop zone having a relatively high surface roughness to improve durability, and data zone having a relatively low surface roughness as compared to the contact start stop zone to improve magnetic recording characteristics. As used herein, a xe2x80x9cread/write head interaction surfacexe2x80x9d means the surface over which the head glides, lands, rests or slides during standard operation of a magnetic recording media system of the type utilizing an air bearing. The data storage of the present magnetic recording media is physically separated from the read/write head landing site, allowing the surface topology of the specialized zones to be individually optimized for either mechanical durability or data storage. Specifically, the relatively high surface roughness of the contact start stop zone exhibits excellent head glide height, stiction, and durability performance, while the relatively low surface roughness of the data zone promotes a low glide height to improve data density, minimize media noise as measured in bit shift errors, and reduce the incidence of certification errors, particularly at higher threshold percentages.
Optimization of the read/write head interface surface of magnetic recording media for both mechanical durability and high-density recording characteristics has been problematic, requiring compromises between competing criteria. In connection with the present invention, it has been discovered that friction between the read/write head and separately optimized contact start stop zone increases greatly when the contact start stop zone has a roughness (Ra) of less than 40 xc3x85. Conversely, surface topologies having a roughness of over 55 xc3x85 suffer head crash at a higher rate than lower roughness surfaces. Mechanical durability is optimized where the contact start stop zone has an average surface roughness in the range between 45 xc3x85 and 55 xc3x85. It has further been discovered that recording density can be increased by limiting the average surface roughness of the data zone to 35 xc3x85 or less. Glide height, certification errors, and media noise are optimized with a data zone surface topology having an average surface roughness in the range between 15 xc3x85 and 35 xc3x85.
In another aspect, the present invention provides improved magnetic recording media of the type having a textured surface. Such textured surfaces are generally imposed on an underlayer or the substrate of the magnetic recording media, typically by abrading an Nixe2x80x94P underlayer with an abrasive tape, a diamond slurry, or the like. The improvement comprises a contact start stop zone on the textured surface having a first surface texture, and a data zone on the textured surface having a second surface texture, in which the second texture has a lower average surface roughness than the first texture. A transition zone extends between the contact start stop zone and the data zone, and has a surface texture which varies from the first texture adjacent to the contact start stop zone to the second texture adjacent to the data zone.
Generally, the transition zone will be textured at least in part during texturing of both the data zone and the contact start stop zone. As with most surface preparation procedures, the surface topology depends on which of the two textures is imposed last on the transition zone. Preferably, at least a portion of the transition zone has the second surface texture imposed over the first surface texture. Thus, the contact start stop zone should then be textured first, followed by the texturing of the data zone. This helps to ensure that most of the relatively rough texture patterns generated by the contact start stop zone texturing process within the transition zone are polished out during the data zone texturing process. Additionally, imposing the data zone texturing over the contact start stop zone texture ensures that stiction performance is not compromised, but cannot guarantee the error performance of the data zone. Conversely, imposing the contact start stop zone texturing over the data zone texture guarantees the error performance throughout the data zone, but compromises the stiction performance. Although optimized magnetic recording media require both maximum stiction and error performance, error performance can be easily and quickly tested on individual disks using a production level certifier. In contrast, verification of stiction performance requires a lengthy testing process. Therefore, it is preferable to texture the data zone last, and ensure error performance by testing.
Generally, the first and second textures are imposed by oscillating the magnetic recording media relative to an associated texturing mechanism so that the surface roughness of the transition zone varies smoothly from the first texture to the second texture. The oscillations of the latter applied texture process must be controlled to within tight tolerances, ideally being controlled to within 0.006 inch to ensure the integrity and alignment of the data zone on the recording surface.
Separately imposing the first texture on the contact start stop zone and the second texture on the data zone typically results in a disparity tin height between the contact start stop zone and the data zone. The transition zone will thus often have a slope, the angle of which depends on the relative difference in heights and on the distance between the contact start stop zone and data zone. In connection with the present invention, it has been discovered that this slope of the texture zone has a significant impact on the likelihood of head crashing. Generally, the slope angle should be as low as possible, ideally being less than 0.004xc2x0.
An improved machine for texturing a zone on magnetic recording media is also provided. The texturing machine is of the type having an abrasive tape and a texture roller for biasing the abrasive tape against the magnetic recording media. The improvement comprises a step on the texture roller having a large diameter relative to a body portion. The abrasive tape rolls against the step and extends beyond the step toward the body portion of the texture roller, and is thereby locally biased against the magnetic recording media in the area of the step. Advantageously, the tape is biased against the recording media only locally at the step, allowing texturing of a limited contact start stop zone without major modifications to the texturing machine. The tape provides a gradual reduction in texturing beyond the step, particularly when the roller is oscillated relative to the recording media.
A machine for texturing magnetic recording media according to the principles of the present invention comprises a magnetic recording media restraint, an abrasive tape, and a texture roller for biasing the abrasive tape against the magnetic recording media. Additionally, an oscillation mechanism is disposed between the recording media restraint and the texture roller. The oscillation mechanism allows adjustments to oscillations with a tolerance of less than 0.006 inch. Typically, the magnetic recording media restraint allows rotation of a magnetic recording disk, while the precise control of the oscillation mechanism ensures the integrity and positioning of the contact start stop zones, transition zones, and data zones of magnetic recording media according to the principles of the present invention. Ideally, the oscillation mechanism has a tolerance of less than 0.0005 inch. Such precise control is available using an eccentric cam with threadably adjustable upper and lower cam followers.
A method for texturing a surface of magnetic recording media according to the principles of the present invention comprises texturing a contact start stop zone with a first texture, and texturing a data zone with a second surface texture. The second texture has a relatively smooth average surface roughness as compared to the first texture.